1. Field of the Invention
The present invention relates to a charged-particle apparatus with a plurality of charged-particle beams. More particularly, it relates to an apparatus which employs plural charged-particle beams to simultaneously acquire images of plural scanned regions of an observed area on a sample surface. Hence, the apparatus can be used to inspect and/or review defects on wafers/masks with high resolution and high throughput in semiconductor manufacturing industry.
2. Description of the Prior Art
For manufacturing semiconductor IC chips, pattern defects and/or uninvited particles (residuals) inevitably appear on a wafer and/or a mask during fabrication processes, which reduce the yield to a great degree. To meet the more and more advanced requirements on performance of IC chips, the patterns with smaller and smaller critical feature dimensions have been adopted. Accordingly, the conventional yield management tools with optical beam gradually become incompetent due to diffraction effect, and yield management tools with electron beam are more and more employed. Compared to a photon beam, an electron beam has a shorter wavelength and thereby possibly offering superior spatial resolution. Currently, the yield management tools with electron beam employ the principle of scanning electron microscope (SEM) with a single electron beam, which therefore can provide higher resolution but can not provide throughputs competent for mass production. Although a higher and higher current of the single electron beam can be used to increase the throughputs, the superior spatial resolutions will be fundamentally deteriorated by the Coulomb Effect which increases with the beam current.
For mitigating the limitation on throughput, instead of using a single electron beam with a large current, a promising solution is to use a plurality of electron beams each with a small current. The plurality of electron beams forms a plurality of probe spots on one being-inspected or observed surface of a sample. The plurality of probe spots can respectively and simultaneously scan a plurality of small scanned regions within a large observed area on the sample surface. The electrons of each probe spot generate secondary electrons from the sample surface where they land on. The secondary electrons comprise slow secondary electrons (energies ≤50 eV) and backscattered electrons (energies close to landing energies of the electrons). The secondary electrons from the plurality of small scanned regions can be respectively and simultaneously collected by a plurality of electron detectors. Consequently, the image of the large observed area including all of the small scanned regions can be obtained much faster than that scanned with a single beam.
The plurality of electron beams can be either from a plurality of electron sources respectively, or from a single electron source. For the former, the plurality of electron beams is usually focused onto and scans the plurality of small scanned regions within a plurality of columns respectively, and the secondary electrons from each scanned region are detected by one electron detector inside the corresponding column. Therefore, the currents or even landing energies of the plural electron beams can be varied individually.
For the latter, a source-conversion unit is used to virtually change the single electron source into a plurality of sub-sources. The source-conversion unit comprises one beamlet-forming means and one image-forming means. The beamlet-forming means basically comprises a plurality of beam-limit openings, which divides the primary-electron beam generated by the single electron source into a plurality of sub-beams or beamlets respectively. The image-forming means basically comprises a plurality of electron optics elements. If each electron optics element is a round lens, as described in U.S. Pat. No. 7,244,949 and shown in FIG. 1A, the plurality of beamlets will be focused to form a plurality of parallel real images of the single electron source respectively. If each electron optics element is a deflector, as described in U.S. patent application Ser. No. 15/065,342, the plurality of beamlets will be deflected to form a plurality of parallel virtual images of the single electron source respectively. Each of the plurality of parallel images can be taken as one sub-source which emits one corresponding beamlet. The beamlet intervals, i.e. the beam-limit opening intervals are at micro meter level so as to make more beamlets available, and hence the source-conversion unit can be made by semiconductor manufacturing process or MEMS (Micro Electro Mechanical Systems) process. In comparison with an electron optics element with conventional sizes, each corresponding lens and deflector are respectively called as micro-lens and micro-deflector or micro-multipole-lens.
In FIG. 1A, three beam-limit openings 21_1, 21_2 and 21_3 of the beamlet-forming means 21 divide one parallel primary-electron beam 2 coming from the single electron source (not shown here) into three beamlets 2_1, 2_2 and 2_3, and three micro-lenses 22_1, 22_2 and 22_3 of the image-forming means 22 respectively focus the beamlets 2_1˜2_3 and form three parallel images 2_1r, 2_2r and 2_3r of the single electron source. The three parallel images are typically real. FIG. 1B and FIG. 1C show one embodiment of the image-forming means 22, which comprises three electric-conduction plates 22-e1, 22-e2 and 22-e3. The upper plate 22-e1 and the lower plate 22-e3 respectively have an upper and a lower large through-round hole and the middle plate 22-e2 has three middle small through-round holes H1, H2 and H3. When the potentials of the three plates are set to form different electrostatic fields above and below the middle plate, each of three middle small through-round holes H1, H2 and H3 will become an aperture lens. In another case (not shown here), the upper plate 22-e1 and the lower plate 22-e3 can respectively have three upper and lower small through-round holes correspondingly aligned with the three middle small through-round holes H1, H2 and H3. When the potentials of the three plates are set to form electrostatic fields therebetween, a round-lens field will be generated along the center axis (such as 22_2_0 of H2) of each of three through-round holes H1, H2 and H3, i.e. three traditional electrostatic lenses with three electrodes are formed.
Naturally, one primary projection imaging system and one deflection scanning unit within one single column are used to project the plurality of parallel images onto and scan the plurality of small scanned regions respectively, and the plurality of secondary electron beams therefrom is focused by one secondary projection imaging system to be respectively detected by a plurality of detection elements of one electron detection device inside the single column. The plurality of detection elements can be a plurality of electron detectors placed side by side or a plurality of pixels of one electron detector. The apparatus therefore is generally called as a multi-beam apparatus. FIG. 2A shows such a multi-beam apparatus 100A with one source-conversion unit shown in FIG. 1A. For sake of simplification, the primary projection imaging system 100A-P is simplified, and the secondary projection imaging system and the electron detection device are not displayed.
In FIG. 2A, the electron source 101 generates a primary-electron beam 102 with a source crossover (virtual or real) 101s, and the collimating lens no collimates the primary-electron beam 102 to be a parallel beam and incident onto the source-conversion unit 120. In the source-conversion unit 120, three beam-limit openings (121_1, 121_2 and 121_3) of the beamlet-forming means 121 divide the parallel primary-electron beam 102 into three beamlets (102_1, 102_2 and 102_3), and three micro-lenses (122_1, 122_2 and 122_3) of the image-forming means 122 respectively focus the three beamlets to form three real images (102_1r, 102_2r and 102_3r) of the source crossover 101s. To image the three real images onto the being-observed surface 7 of a sample 8 with small aberrations and therefore form three probe spots (102_1s, 102_2s and 102_3s) thereon, the primary projection imaging system 100A-P basically comprises one transfer lens and one objective lens. To reduce off-axis aberrations, the transfer lens can be placed to function as a field lens (U.S. Pat. No. 7,244,949) or form the telecentric path on the sample side of the objective lens (U.S. Pat. No. 7,880,143).
Two key issues limit the available performance and application conditions (currents and landing energies of the plural beamlets) of this multi-beam apparatus as one yield management tool. The first one is the difficulty of changing currents of the plural beamlets or the probe spots, and the second one is the non-uniformity of sizes of the plural probe spots due to off-axis aberrations generated by the collimating lens and the primary projection imaging system. Some samples require specific currents of the plural beamlets due to charging-up, and the first issue may make observing such samples impossible. Due to the second issue, the differences of the image resolutions of the plural small scanned regions may increase detection errors of some defects.
As shown in FIG. 2B, obviously, the current of the plural beamlets can not be changed by varying the focusing power of the collimating lens 110. If the focusing power is weakened or strengthened, the primary-electron beam 102 will become divergent or convergent accordingly. In these cases, the off-axis beamlets 102_2 and 102_3 (not along the optical axis 110_1 of the collimating lens no) will be not parallel to the optical axes 122_2_1 and 122_3_1 of the corresponding micro-lenses 122_2 and 122_3. Accordingly, the corresponding images 102_2r and 102_3r will have radial shifts ΔP2 and ΔP3 with respect to the optical axes 122_2_1 and 122_3_1. The radial shifts ΔP2 and ΔP3 depend on the off-axis distances P2 and P3 respectively, and consequently incur non-uniform pitch variations of the plural probe spots on the sample surface 7. This will generate undesired gaps or overlays between adjacent scanned regions, and therefore reduce the throughput and deteriorate of the resolutions due to additional cross-talks of the images thereof.
Certainly, the current of the plural beamlets can be changed by varying either the emission of the single electron source or the sizes of the beam-limit openings (US2013/0,187,046). The single electron source takes a long time to become stable when the emission thereof is varied. To change the sizes of the beam-limit openings, the beamlet-forming means needs to have more beam-limit openings with different sizes. It is very time-consuming for moving and aligning the beam-limit openings with desired sizes.
Regarding the second issue, the off-axis aberrations will change with respect to the operation conditions of the primary projection imaging system. As well known, the landing energies of the plurality of probe spots and/or electrostatic field on the sample surface 7 are usually chosen according to the features (such as material and pattern sizes) thereof, hence the operation conditions need to be adjusted correspondingly. Among the off-axis aberrations, as proposed by U.S. Pat. No. 7,244,949, by specifically arranging the size differences, shape differences and position differences of the micro-lenses (122_1, 122_2 and 122_3 in FIG. 2A), the field curvature aberrations, the astigmatism aberrations and the distortions can be compensated. However, these size differences, shape differences and position differences can compensate the off-axis aberrations for certain landing energies but may be not competent for some others. Therefore the acceptable range of landing energies and the number of beamlets may be limited.
Accordingly, it is necessary to provide a multi-beam apparatus which can simultaneously obtain images of a plurality of small scanned regions within a large observed area on the sample surface with high image resolution and high throughput in variable application. The multi-beam apparatus is especially needed to match the roadmap of the semiconductor manufacturing industry.